We have used quantitative ligand binding studies, to characterize the receptors for (+)-N-allyl-normetazocine (designated (+)-SKF 10,047), phencyclidine (PCP), and several phencyclidine analogs in membranes from rat and guinea pig brain. We found that rat brain has one class of sites for SKF 10,047, and two for PCP. In guinea pig brain, there appears to be 2 classes of sites for SKF (only one is suppressible by haloperidol), and 2 classes of sites for PCP. Demonstration of these sites required the use of computer modelling, using the LIGAND program, to examine modelling, surfaces, blocking experiments, and for simultaneous analysis of results from multiple curves, multiple experiments, and multiple labeled ligands. We have continued studies of subtypes of mu opioid receptors in brain, and of kappa opioid receptors in bovine adrenal medulla. We have demonstrated that the vasopressin receptors present in high concentration in porcine seminal vesicles are indistinguishable from the V2 receptors of porcine renal medulla, and these receptors have been localized to the epithelium and not to the musculature. We are now examining the vasopressin and oxytocin receptors of male and female genital tract in the human.